Harmful substances such as soot dust, sulfur oxides (SOx), and nitrogen oxides (NOx) are contained in flue gas emitted from combustion facilities such as a boiler and a waste combustor and need to be removed by using a flue-gas treatment device. A typical flue-gas treatment device includes a denitrator that reduces NOx and a wet desulfurizer that uses an alkali absorbent as a SOx absorbing agent. The flue-gas treatment device treats harmful substances contained in flue gas by supplying ammonia (NH3) on an upstream side of the denitrator in a flue gas duct to reduce nitric oxide (NO) with a denitration catalyst of the denitrator to remove NOx as shown in the following formula (1) and absorbing SOx in the alkali absorbent by using the wet desulfurizer (see, for example, Patent Literature 1).4NO+4NH3+O2+4N2+6H2O  (1)
SOx includes SO2 and SO3. When a gas temperature of the flue gas reduces, SO2 and SO3 may change into ammonium hydrogen sulfate or ammonium sulfate as shown in the following formulas (2) and (3), and attach to the wall surface of the flue gas duct or devices that are installed in the flue gas duct. The ammonium hydrogen sulfate, the ammonium sulfate, and the like may cause clogging of an element of an air heater due to attachment thereto. Furthermore, the ammonium hydrogen sulfate and the ammonium sulfate are corrosive substances and thus the wall surface or the devices may corrode when these substances are attached thereto.NH3+SO3+H20=NH4HSO4  (2)2NH3+SO3+H2O═(NH4)2SO4  (3)
To reduce NOx, NH3 is supplied in the upstream step of the denitrator. However, NH3 is used for neutralization of SO3 and thus the supply amount of NH3 also needs to be adjusted.
Accordingly, gas analysis methods that enable to extract a part of flue gas, and perform a ultraviolet absorption analysis to analyze SO3 and NH3 in the flue gas and to measure concentrations of SO3 and NH3 in the flue gas have been conventionally proposed (see, for example, Patent Literature 2).
Coal-combustion flue gas or flue gas produced when heavy oil is combusted may contain metallic mercury (Hg0) in addition to soot dust, SOx, and NOx. Recently, methods or devices that enable to treat the metallic mercury (Hg0) by using a combination of the denitrator that reduces NOx and the wet desulfurizer that absorbs SOx have been variously developed.
As examples of the method that enables to treat the metallic mercury (Hg0) in flue gas, methods of spraying an NH4Cl solution in a liquid form on the upstream side of a reduction denitrator in a flue gas duct to supply the solution into the flue gas duct are proposed (see, for example, Patent Literatures 3 and 4). When the NH4Cl solution is sprayed in a liquid form into the flue gas duct, NH4Cl dissociates into ammonia (NH3) gas and hydrochloric acid (HCl) gas. The NH3 gas acts as a reductant and the HCl gas acts as a mercury chlorinating agent. That is, on a denitration catalyst filled in the reduction denitrator, NH3 has a reduction reaction proceeding with NOx in the flue gas as shown in the formula (1) and HCl has a reduction reaction proceeding with Hg0 in the flue gas as shown in the following formula (4). After NH3 is reductively denitrated on the denitration catalyst and the metallic mercury (Hg0) is oxidized to an aqueous mercury chloride (HgCl2), HgCl2 is dissolved with water by a wet desulfurizer installed on the downstream side to remove mercury contained in the flue gas, and SOx contained in the flue gas is absorbed and removed.Hg0+½O2+2HCl→HgCl2+H2O  (4)